FROM DILETTANTE TO DILIGENT EXPERIMENTER: AREAPPRAISAL OF LEEUWENHOEK AS MICROSCOPISTAND INVESTIGATOR

Brian J. Ford

Antony van Leeuwenhoek(1632-1723) remains one of the most imperfectlyunderstood figures in the origins of experimental biology. The popular viewis that Leeuwenhoek worked in a manner that was essentially crude andundisciplined,using untried methods of investigation that were lacking in refinement
and objectivity. He has often been designated as a "dilettante". His
microscopes, furthermore, have been described as primitive and doubt hasbeen expressed over his ability to have made many of the observations attributed to him. Recent research shows these views to be erroneous. Hiswork was carried out conscientiously, and the observations were recorded withpainstaking diligence. Though we may see evidence of his globulistunderstanding of organic matter (and indeed, this view has frequently been cited as evidence of his observational inadequacies), this minorpreoccupation
cannot detract from two firm principles that underlie his work: (a) a
clear ability to construct experimental procedures which were, fortheir time, rational and repeatable, and (b) a willingness both to fly in theface of received opinion - for example, over the question of spontaneousgeneration - and to abandon a previously held belief in the light of new evidence.

In his method of analysing a problem, Leeuwenhoek was able to laymany of the ground rules of experimentation and did much to found, not onlythe science of microscopy, but also the philosophy of biological experimentation.[1] At the time of his entry into the world of observational microscopy,Leeuwenhoek was already following several noted predecessors. We may citeexamples:

HUMAN HISTOLOGY

Capillary circulation in the lung (Malpighi, 1661)

Hematology and general organ microscopy (Malpighi, 1665-6)

ENTOMOLOGY

Insect morphology (Power, 1664; Hooke, 1665)

Anatomy (Swammerdam & Malpighi, 1669)

BOTANY

Plant Histology (Grew, 1672; Malpighi, 1675, 1679)

EMBRYOLOGY

Rana (Swammerdam, 1669)

Gallus (Malpighi, 1673) [2 ]

However, it was typical of these earlier
workers that they confinedthemselves
largely to microscopical investigation, with some small degree ofpreparative technique (injection with air, for
example, or the dissection ofmaterial).
Leeuwenhoek assuredly entered the field with purely observationalmicroscopy, too; but in his developing understanding
we may perceive theorigins of
experimental biology. Leeuwenhoek's birthdate might seem to categorise him as a
seventeenth centuryinvestigator. Had he
begun work at twenty years of age and retired at sixty-five, his active years
would have been 1652-1697. It is noteworthy that hedid not start his microscopical activities until he
was forty. On his deathbed, aged over
ninety, he was still dictating new observations. Thus, hisactive period covers 1673-1723 and many of his
observations set in train theexperimental
work of the 1700s.

Documentary evidence of Leeuwenhoek's
interest in experimentation may be found in the letters in which he recorded
his work. The bulk of these were sent tothe
Royal Society of London, and are preserved in the Society's rooms inCarlton House Terrace. [3]Leeuwenhoek's first letter was sent to Royal Society
by de Graaf in 1673. Themanuscript is
lost. It appeared in Philosophical Transactions, VIII (94),6037-6038 as: A Specimen of some Observations made by
a Microscope, contrived by M. Leeuwenhoek in Holland, lately communicated by
Dr. Regnerus de Graaf;the illustrations
were published later, in VIII (97), 6116-6119. In it hetook the unnamed Robert Hooke to task for some of the
descriptions publishedin Micrographia,
1665, Martyn and Allestry. It is clear that Leeuwenhoek was much influenced by
this tome, which had been a talking-point during his(only) visit to London in 1668. The citing by
Leeuwenhoek of the samespecimens as
those described by Hooke, and in the same numerical order, is aclear indication of the connection. The chances of a
random selection of specimens proving to be coincident are worse than 11023, given thepossible number of species from which material
could be selected. Theargument that
Leeuwenhoek had been told the kind of material of interest tothe Fellows of the Royal Society is not convincing,
either; even here the chances are remote that the three Leeuwenhoek would elect
to send would beidentical with those
categorised by Robert Hooke - and in the same order oflisting.

Within a short time, Leeuwenhoek was
beginning - not merely to observe - but to experiment.His earliest examples of specimen preparation date
from the letter of 1 June1674. Here he
prepared fine sections of elder pith, and cork, and enclosedthese in a folded envelope for the Secretary of the
Society, Henry Oldenburg, and his 'curious friends' to observe. [4] By 7
September 1674 he was working on the anatomy of the eye throughdissection, and on 4 December of that year he
described work on the optic nerve (specimens of which I have found still
survive, and were amongst thosedescribed
elsewhere, supra. The drawing sent with the letter of 26 December1674 is preserved at the British Museum under
Additional Sloane MSs folio125, and the
letter with which the specimens of optic nerve were sent toLondon was dated 22 January 1675. Thus, within less
than two years ofstarting work,
Leeuwenhoek was sensing the need to evolve technical methodsin his research, and was beginning to move towards
practical experimentation.Leeuwenhoek
was elected to Fellowship of the Royal Society on the strength ofhis pioneering investigations. He was not alone in
being so honoured, though aforeigner:
Giovanni Domenico Cassini,Christiaan Huygens, G. W. Leibnitz,Marcello Malpighi and Vincenzo Viviani came into the
same category.

What marked out Leeuwenhoek was his
determination to work independently. There had been nothing in his background
to suggest any form of philosophical trainingor educational specialisation. His parents had been Phillip - a maker
of thewicker baskets used to transport
the fine wares produced in Delft - and abrewer's
daughter, Margaretha. Following the death of his father the young Leeuwenhoek
was sent away to be educated and in due course he was apprenticedto a cloth merchant. The young Jan Swammerdam was in Amsterdam
at the sametime as Leeuwenhoek was
serving his time in that prosperous town, though wecould not deduce that there was any
cross-fertilization of ideas. Leeuwenhoek was sixteen, whilst Swammerdam was
only eleven. Neither should we be tooeasily
persuaded that Leeuwenhoek's interests in lenses arose from the clothcounters - lenses used to measure the density of warp
and weft in the textiles which he graded - since there is no evidence that
Leeuwenhoek becameinterested until he
was intrigued by Hooke's already published descriptions. The accounts from 1673
describe Leeuwenhoek's microscopes as being of recentinvention, furthermore.

Once qualified, Leeuwenhoek went to work for
a Scottish trader, WilliamDavidson, who
dealt with the East Anglian textile merchants. At the age of22, Leeuwenhoek returned to Delft and purchased a
house, The Golden Head, inwhich he lived
for the remainder of his life. Leeuwenhoek's mother died in1664, his own wife Barbara died in 1666, and it was in
this same year that he took on the job of Chamberlain of the Council Chamber.
This honour, somewhatakin to being
'master of the king's bedchamber', as Elmer Bendiner hasfittingly said [5] offered him a degree of civic
status and a permanent sourceof income.
With this (and the inheritance from his mother's estate) he was able to
concentrate on his investigations. His marriage in 1671 to CorneliaSwalmius (daughter of a merchant who dealt in serge,
and a distant relativeof his first wife
Barbara) brought him into contact with a more intellectualgroup and within two years he had embarked on his
life's work as a microscopist.

I have elsewhere drawn attention to the
distinction that should be drawnbetween
Leeuwenhoek's microscopical investigations and those of hiscontemporaries. They, typified by Robert Hooke, were
concerned with the magnification of the already-familiar: fleas and lice,
nettles and beestings.But Leeuwenhoek
was concerned with the previously invisible andunsuspected: the globules in milk, erythrocytes in the
bloodstream, and microbiota of ponds, lakes and streams. Though he was not (as
Bendinersensibly emphasised in the title
of his paper) the inventor of themicroscope,
he was the father of high-power microscopy and the progenitor ofmicrobiology. Little attention need be paid to the
oft-repeated argument that Leeuwenhoekwas
unable to read English, since he was a self-confessed monoglot speaker ofthe Early Modern Dutch of his time. As modern
workers know very well, it isalways
possible to find some friendly individual able to render into one'smother tongue a publication written in a foreign
language, and Leeuwenhoek was known to consult translators when he needed them.
In his letters hecited works published
by philosophers who did not communicate in Dutch; theyincluded Willis's CerebriAnatome (1645), the
1665/7 Micrographia of Hooke(to
which allusion has been made earlier), Swammerdam's Historia Insectorum
Generalis of 1669, Redi's influential Experimentacirca GenerationemInsectorum (1671), de Graaf's De Mulierum Organis Generationi
(1672), Grew'sComparative Anatomy of
Trunks (1675), and Willoughby's Historia
Piscium of1686. His knowledge of
many of his contemporaries was considerable, and hislack of ability to translate was manifestly no
insurmountable obstacle to hisdesire to
know what others were publishing.

Let us examine how this links Leeuwenhoek
with eighteenth-century experimental biology. An examination of the status of
his investigations asthat century begins
sets his work in a clearly forward-looking context. Hebegins his eighteenth-century work with a letter
written to Sir Hans Sloanein London and
dated 2 January 1700. In it, Leeuwenhoek describes the colonialflagellate Volvox. He reveals the delicacy of
structure, the crystallinebeauty of the
organism, and its characteristic method of locomotion, rollingthrough the aquatic environment, propelled by the
coordinated beating ofcilia of the
daughter-cells. But he then moves on to evolve an experimentalrationale that enables him to study the reproductive
mechanisms of this colonial organism. He selects two of the colonies and traps
them in a narrowglass tube. One end of
the tube he closed with cork, leaving a region of airbetween the cork and the column of water. Even before
Leeuwenhoek examines the organisms which are the centre of this study, he is
writing observationson the system he has
thus created: "One cannot approach the tube with thehand, the breath, or any other part of the body that
is a little warmer thanthe air now shut
inside the tube, without the air being affected by some partof it ... even though we may perceive no motion with
the naked eye".From this colloquial account of a primitive thermometric
mechanismLeeuwenhoek moves on to
describe in detail the behaviour of the volvocine colonies he has separated
from the rest of the culture. As he expected, smalldaughter-colonies were released from within each of
the mature spheres ofcells. He continued
his observations by writing a daily diary of theprogression of these newly-released colonies towards full maturity. In
this we may see his preoccupation with the defeat of a concept of spontaneousgeneration; to Leeuwenhoek it always seemed
obvious that microbial organismsmust
have parents, rather than being derived from inorganic matter, and inmany of his later experiments he produces evidence in
support of his sensible contention. During this work he recorded that the
rotifera in the aquaticsamples from
which he had obtained his Volvox colonies contained "redmaterial in their guts" and he related this,
correctly, to the free-livingHematococcus
which he studied at some length.In noting that the red coloration of the
rotifera was derived from theirconsumption
of the free-swimming red algae, Leeuwenhoek recorded that "some of these
animals ... had none of the red material in them, particularly theyoung ones which had not long left their mothers'
body".

He also studied Chlamydomonas, a
green coloured free-swimming organism, and here inoculated cultures into
samples of water that were free of such contaminants. Hisaccounts show that he transferred the cells to both
fresh and boiled water:do we here see an
anticipation of the "vital force" concept of later years?On Christmas Day 1702 Leeuwenhoek made his discovery
of the sessile ciliateVorticella, "fashioned like a bell, and at the round
opening (making) such astir that
particles in the water thereabouts were set in motion ..."Leeuwenhoek, it should be noted, considered himself a
poor draughtsman and utilized the services of a limner to assist in recording
his observations.His account of rotifera
adds: "Suddenly there came out its roundness twolittle wheels, which displayed a swift rotation. The
draughtsman, seeing thewheels go round
and round, and always turning in the same direction, couldnot have enough of looking at them, exclaiming, 'Oh,
that one could ever depict so wonderful a motion'!". During these
observations Leeuwenhoek alsorecorded Hydra
[6] for the first time. During these years Leeuwenhoek witnessed the apparent
ability of certainaquatic organisms
(notably the rotifera) to survive periods of dessication.In his letter to the Society dated 5 November 1716 he
wrote of a culture hehad left dry
"for a whole winter" and recorded that: "when I put some of themin water I saw them unfold their limbs, which
seemed to be wrapped up insidethem, and
swim about."It was also in 1700
that Leeuwenhoek successfully undertook an experiment onthe parasitism of aphids. Endoparasitic organisms were
viewed byLeeuwenhoek's contemporaries as
examples of the mysterious workings of acreator
[7] and it is noteworthy that, although Redi had by this time shown thatdipteran species were not produced spontaneously,
even he was not drawn to auniversal view
that all life arose from living progenitors (clearly the opinion of Leeuwenhoek
at this time).In 1678 Leeuwenhoek noted
the apparent emergence of a fly (Cole assumes thisto have been a hymenopteran parasite) from the cocoon
of a caterpillar. Eight years later he set up experiments to isolate insects
emerging from the gallsof oak and
thistle, and began the study of galls of willow. He showed that inthe first two cases, the larva of the insects (Cynipsfolii
and Urophorascardui respectively) caused the trauma to the host plant
from whicheventuated the gall itself.
Though he noted the development of the larva andpupa within the gall mass, he did not complete the life-cycle of either
species. By 1695 he had watched the hatching of Chalcid parasites from theapple ermine moth Hyponomeuta, and
during the same period he was firstacquainted
with parasitism in the aphids. His observations were stimulatedfirst by the observation of some empty exoskeletal
structures of aphids, eachpunctured by a
neatly bored hole (through which, as he rightly surmised, the parasite had
emerged). He went on to examine a number of turgid and immobileaphids in which he found the entire body cavity was
taken up by a larva. Hisfirst conclusion
was that a female 'ant' had laid eggs which hatched toproduce the voracious maggots he observed. By 1696 he
had shown that the apple sawfly Hoplocampa was the origin of larvae
found inside those fruit,and that the
parasite now known as Therioaphis may
be hatched from infestedspecimens of Tilia,
the lime.In 1700 he succeeded in
demonstrating the completion of the cycle, throughobservation of the breeding behaviour in adult
parasites. His use of a confined chamber for the isolation of his specimens
enabled him to observethe hatching of
flies from the bodies of parasitised aphids. Of fundamentalimportance was his subsequent observation of
oviposition, and he clearlynoted the
fact that copulation in these flies did not precede the egg-laying phase. He
went further, and confined adult newly-hatched parasitic flies withlepidopteran caterpillars, noting that the parasites
could not be induced tolay eggs on the
(alien) host species. He wrote that the ovipositor wasproduced "in the manner of a sting" and was
used to inject the eggs into thehost. During
the following year he observed two parasites within a willow gall, and showed
that the smaller larva was parasitic upon the larger. Hisdescriptions of the organisms and their microscopical
structure are accurateto a degree; he
was able to demonstrate experimentally that the smaller larvafeeds upon the larger, and that both mature to form
disparate species. Hiswork on parasitic mechanisms
laid the firm foundations for this previously unexplored area of biology, and
the experimental techniques which he utilizedlaunched parasitology as a philosophical discipline.

During the late 1690s, Leeuwenhoek advanced
the view that the annularstructures
observed on fish scales corresponded to the age of the fish inyears. By 1716 he was diverted by the common carp, Cyprinus,
and required ameans of examining the
layering configuration in greater detail. To do thishe evolved the technique of sectioning the scales at
an angle, thusexaggerating the apparent
thickness of the separate layers. His experimental approach involved soaking the
scales in water for long enough to soften thestructure, and then cutting sections at an extremely oblique angle.

Similartechniques
have been used in the succeeding centuries to clarify laminarstructures, and one modern application is the forensic
analysis of multi-layered painted surfaces.Leeuwenhoek
was rightly fascinated by image-forming structures apart from lenses. In 1674
he had reported producing an image using, as a lens, the ovum of a cod. Twenty
years later he generated clear images of a candle-flamethrough the compound corneal structures of the
dragonfly Libellula. By 1700he had reported on the optical arrangement of the
centipede Scolopendra andwisely
concluded that the compound eye he painstakingly dissected did notprovide multiple images, any more than a human has
double vision through thepossession of two
eyes. He used his ability in microdissection to examine the ommatidia of Crangon,the
common shrimp, and dissected out the crystallinecones using a hand-held needle. That most entrancing of microscopical
subjects, Daphnia, came to hisattention
in the same year. Leeuwenhoek had carried out extensive studies onblood circulation during the seventeenth century, and
now recognised that thecontractile vescicle
he observed in the thorax of the water-flea was its heart. His earlier
descriptions of erythrocytes frequently alluded to theirbeing composed of smaller globules. Possibly he had
observed cells undergoingcrenation, in
which rounded projections appear to form on the surface of eachcell as the cytoplasmic volume decreases. His
observations of the blood cellsof the flounder include the crucial comment that
each ovoid cell contained aclear central
zone. This, through the work of Robert Brown in the 1820s,proved to be the cell nucleus [8].He seems to have
repeatedly concluded that erythrocytes were discoid or oval,yet returns time and time again to a globulist view -
and at one stage during1700 he proposed
that human erythrocytes might be composed of 36 globules(six component spheres each comprising six sub-units).
He even produced waxspheres which he
packed together and described in detail, in an attempt to support this globalist
proposition. The fact that his experimental procedures did not offer the
confirmation he sought did not, at the time, deter him fromcontinuing this largely groundless speculation. The
detailed examinations he made in that year ranged from the newt Tritonto the lizard Lacerta; it covered a
range of fish and even the spider Araneus.By1708 he had experimentally removed the living heart
from a small eel,Anguilla, and maintained it beating for four hoursin vitro.
His skill atmicrodissection also enabled
him in 1700to dissect from the queen honey-beeApis a vast
number of immature eggs, and to show that the 'king' bee, as itwas then known, was no 'king' at all, but a queen.

The most compelling version of Leeuwenhoek's
introduction to themicroscopical
universe is contained in the compelling biography of CliffordDobell, whose widow Monica has broadened and deepened
my understanding of thepublished account
[9]. Leeuwenhoek described microorganisms including algae, protozoa, rotifera
andbacteria in fresh water samples and
recorded that: "The motion of most ofthem
in the water was so swift, and so various, upwards, downwards, and roundabout,
that I admit I could not but wonder at it. I judge that some ofthese little creatures were above a thousand times
smaller than the smallestones which I
have hitherto seen on the rind of cheese, wheaten flour, moldand the like".Similarly, from his letter on pepper-water described by R. T. Gunter's Early
Science in Oxford,(1931) VIII, 299: "Some of these
are so exceedingly smallthat millions of
millions might be contained in a single drop of water. I wasmuch surprised at this wonderful spectacle, having
never seen any living creature comparable to those for smallness; nor could I
indeed imagine thatnature had afforded
instances of so exceedingly minute animal proportions".Yet he remained intrigued by the conventional behaviour
of more familiarspecies. On 12 October
1685 he wrote on his observations of seeds. Hisdescription of the cassia seed causes him to consider an experimental
means of confirming his observational conclusions:

"As regards the cassia seed, I find in
it the beginning of a plant; that is, chiefly the leaves, which, I trust, have
been made so exceedingly large inorder
to provide nourishment for that part of the root and for the beginningof the young plant; which root, by comparison with the
two leaves, is veryshort. In order to satisfy
myself on this point, I have laid the cassia seedto sprout in sand moistened with common rain-water,
until the root had grown as long as the width of my thumb, when the two
aforesaid leaves had beenpushed outside
the earth, having between them the beginning of the youngplant, which before that could not be discerned."

He adopts an innovative approach for the
examination of the internalstructures of
the cotton seed, a topic addressed in his latter of 2 April1686:

"I have thought fit to put some cotton
seeds - which I have had by me for over a year, and which are so old that their
greenish colour has already faded - in water for one night, after which I
removed from them their tough rind,being
their first; and then their soft membrane, being their second envelope;and separated the leaves a little from one another.
Eight or nine of theseseeds, from which
the young cotton tree takes its origin, I send youherewith. On these a sharp eye will recognise, even
without any magnifying lens, not only the four distinct leaves, together with
that part which willbecome the root and
stem; but one will also be able to see the small spots onthe leaves [10]."He derived a clearer view of the internal structure of these seeds by
the useof a technique latterly known as
serial sectioning. Leeuwenhoek took some ofthe
soaked cotton seeds and "cut one of them into twenty-five to twenty-sixround slices, and the other into twenty-eight to
twenty-nine round slices,which too I
send you herewith."

The Collected Letters published a
footnote to this (Volume VI p 11) to theeffect
that the 'slide' Leeuwenhoek sent to the Royal Society is "no longerin the library". At the time, a 'slide' would
not, of course, have existed;and the
correspondent was equally wrong to imagine that the material wasmissing. The translations were done from microfilm
copies of the original letters, and the small packets which Leeuwenhoek had
used to contain hisspecimens were pasted
adjacent to his signature. As I have explained earlier,vide: Notes and Recorded of the Royal Society,
(1981), 36 (1), 37-59, thepackets
were present all the while, but the image they presented to thecamera deluded the translator into thinking Leeuwenhoek
had drawn'rectangles' at the end of his
account. The images were, in fact, the outlines of the folded paper packets,
unopened for three centuries. The useof
the term 'slices' draws a neat distinction between these portions of plantmaterial and the fine sections which Leeuwenhoek
had earlier prepared of corkand elder
pith, q.v. and his decision to introduce the concept of the serialtechnique has had many later examples in the realm of
experimental biology.Leeuwenhoek may be
credited with the establishment of a pioneering example offorensic microscopy. At the time of his active period
it was believed that'heavenly paper'
descended from time to time to the earth's surface, asthough messages from a divine source. The charred
appearance of the samplesalmost suggests
an early anticipation of the heat of re-entry! Some small fragments were
collected by one of Leeuwenhoek's correspondents in Courlandon the Baltic coast. They took the form of blackened
fragments of a paperysubstance and were
safely contained in a square of paper folded over fourtimes.(An identical method of enveloping has come down
to the present timein the field of
gemmology, for such folded containers are used to hold precious stones during transportation).
The handwriting on the envelopesuggests
that the correspondent was not accustomed to writing in Dutch, andwas possibly unfamiliar with the Latin alphabet.Leeuwenhoek examined the specimens when they
arrived at his home in Delft. Inthe
letter dated17 October 1687 he wrote:

"I had not had this supposed paper in my
house for half an hour before I had (with the aid of the microscope) formed
such a clear impression of it, that Iconcluded
it was a plant which had come forth from the water. And moreover, Itook it for sure that, if it were true that it had
fallen from the sky ontothe field, then
this substance must first have been driven up into the air(by a cloud which we
call a whirlwind). But I much prefer to believe that,due to heavy rains or melting snow . . . the water
from a marsh or from ditcheshad flooded
some piece of land, and that the water had left this green plant,from which the supposed paper is made, behind on a
greensward or a field withyoung corn,
with the result that the sun and wind caused the plant to becomedry and stiff, so that it took on to some extent the
look of burned paper."

The original specimen packet was sent by
Leeuwenhoek to London with thisletter,
and indeed the 'heavenly paper' was seventh of the nine specimenpackets which I found hidden amongst his correspondence.
It should be clearlystated that the
appearance of the fragments was that of charred paper; andwhen a small portion was gold sputtered and examined
under the scanning electron microscope the first impression was that of a paper
sample. Atechnician whose previous
experience had been in the paper manufacturingindustry offered this as an immediate diagnosis on seeing a
low-magnification scanning image of the material. Leeuwenhoek's abilities as a
microscopicalanalyst are here thrown
into clear perspective: he was making a judgementwhich might compare favourably with routine forensic
examination today. And inthis case too,
the investigation led Leeuwenhoek to an experimental modelling of the processes
which had produced the 'paper'.

His conclusion was clearly that the 'plant'
material, as he designated it,had
originated as a mass of algal growth floating in water. He emphasisesthis in the following extract from the letter of 17
October 1687:

"I concluded that I had often seen this
substance in large quantities in stagnant waters, such as ditches and excavated
ground; but what puzzled mewas how I could
possibly make this Substance, or green plant, turn into ablackened mass. This green plant is often called felt,
but more often slime,by the common
person."

From this he proceeded to elaborate a simple
technique for demonstrating theproduction
of 'paper' from the slimy growth of chlorophyte algae. His firstimpulse was to go out of Delft to the drains which
abounded in the low-lyinglands, but he
soon realised he had an alternative supply nearer at hand, andavailable as a result of the Dutch expertise at managing
water flow. Theletter continues:

"I bethought myself to go to some swampy
fields, situated not far from our City; but on reflection that the canals which
run around our City havesluice-gates in
two distinct places, in order that the daily current of watershould run, not around, but through the City, I went
to where the water inthe City canal had
least movement, and where I saw the slime in abundance. [11] Of this slime I
have taken some and laid it on several pieces of thickpaper, and dried the same in front of the fire; and I
saw that where it layvery thick, it
changed by itself from a clear green into a blackish substance; and where it was
quite thin it retained its green colour.Furthermore,
I once again examined the so-called burnt paper, and now I sawvery distinctly that it was one and the same
substance, and of the samecomposition. For,
when I examined the green substance, just as I had taken itfrom the water, through an ordinary microscope, I
imagined seeing that these very thin, thread-like parts, which by far exceed a
hair in thinness, wereround, and that
their membrane was very transparent, and that they werefilled with green globules . .".

His account adds that he observed 'joints' inthe filaments. These were the transverse septa
which divide one cell from thenext
inline.Subsequently Leeuwenhoek
collected a further sample of algal material from abarrel of water kept to irrigate a small garden in
Delft, and dried it down in a similar fashion before the fire. In this case, he
wrote, the paperysample which resulted
retained its green coloration. Of this material, andthe papery specimen produced in the experiment
described above, he sentsamples to the
Royal Society. The packets were among those I found to have survived intact,
and both were made available for microscopical examination.Here we have an anticipation of conventional
experimentation. Leeuwenhoekfirst
utilizes his microscope to correctly diagnose the specimen material: itis not the paper it appears, but, rather, a sample of
dried algal growth. Hethen moves on to
recreate the original conditions which he postulates gaverise to the specimen. Deciding at first to employ
algal material from fields like those in which the original material had been
found, he decides insteadto opt for more
readily-available material in the City canal system. Hesuccessfully imitates the process which had been
postulated to produce the result observed. And thirdly, he moves on to a
separate source of materialand repeats
the procedure. His first experiment produces a dark-colouredpapery specimen; the second produces a 'paper' which
is green in colour butotherwise similar.
By way of confirmation we see him comparing the results with correlated
observations of the original sample. The methodology isempirically developed; it comes close to anticipation
of the controlledexperimentation employed
in more recent times.Investigations of
the material have shown how accurately Leeuwenhoek obtained results and
interpreted his findings. Interestingly, the fact that he drieddown freshly gathered aquatic algal matter implied
that - incidentally - hewas bequeathing
to a twentieth-century microscopist dehydrated samples of thevery material which he studied during his
microscopical research. Carefulreconstitution
of the dried material has restored many of the organisms to a near life-like
appearance, and has enabled us, for the first time, toidentify in Leeuwenhoek's own material some of the
aquatic types he describedin his
letters.

Leeuwenhoek has been described as possibly
the first person to preparesections for
the microscope. That clearly cannot have been the case; Hookeportrayed fine cork sections in his Micrographia and this was work
undertakena decade before Leeuwenhoek's
entrance upon the stage. However, he continuedto apply his techniques to an unprecedented range of specimens. In the
fieldof histology he demonstrated, in
1706, the fibrous capsule of the spleen and its trabeculae, pulp and corpuscular
structure. He studied striated(voluntary)
muscle, the structure of the eye and even (in 1720) sections ofbone. Here too he introduced experimental methods, for
in 1714 he referred tothe transparency of
muscle tissue and found how to stain the fibres with a solution of saffron in brandy.

In 1688 he had made his crucial observations
of the capillary circulation in the tail of the tadpole stage of Rana;
by 1700 he was able to document the phenomenon of blood coagulation. During the
eighteenth century, Leeuwenhoekundertook
accomplished dissections of insect, crustacean and mammalianspecimens. For example, he employed microdissection in
his studies of the cochineal bug Coccus
cacti, in which he demonstrated pre-formed insectswithin egg pouches removed from adult females. He
carried out experiments onfumigation
with sulphur dioxide, which he produced by burning flowers ofsulphur. When moths reappeared in a warehouse he had
treated, he rightly concluded they had emerged from chrysalids which - because
of their structure- had resisted the
effects of the gas; he therefore evolved a scheme toensure that a follow-up treatment was timed to destroy
these. He observed thedetonation of
gunpowder, compromising his eyesight as he did so, anddiligently recorded the effects of aromatic substances
as insect repellents and pesticides. His single-mindedness meant that
Leeuwenhoek - who showed endless patiencewith
his microscopic subjects - was less concerned with social niceties.

Though visited by Queen Mary of England and
the Czar of all the Russias, Peter the Great, he often had distinguished
visitors turned away if they hadnot made
an appointment and he felt disinclined to socialize that day. Henever read a research paper, taught a student, nor
visited a University. Notonly was he
disinclined to teach, but wrote that he was unwilling to bechallenged on his findings.He has been taken to task for a failure to associate
microorganisms with thegeneration of
infectious disease, but in my view this assures him of a reputation for
far-sightedness. Our microbial compatriots have been viewedfor too long as agencies, primarily, of disease; a
better understanding oftheir role is to
view them as the seat of environmental regulation, thesource of the complex mechanisms which provide our
food and our atmosphere, and as objects of incomparable complexity and
considerable enchantment. If wewere to
abandon the view that 'microbes' were synonymous with 'germ', andthat 'germ' implied 'disease', I believe science could
develop a fuller understanding of the complex interactions which manifest
themselves as life.In a primitive way,
but with remarkable prescience, Leeuwenhoek was sensitiveto this cause. His delight in making new observations,
in piecing togethernew levels of
biological understanding, have given him a unique role in the formation of a
scientific approach at the very dawn of the discipline.

Thereremained
an attitude that Leeuwenhoek was an outsider, a notion stemming inpart from what has been described as intellectuals
turning "against Leeuwenhoek". [12] But Leeuwenhoek's abilities are
clear from the documentary record and,latterly,
are extended by our new knowledge of the exemplary specimens he prepared in the
later seventeenth century. He should not be seen merely as anobservational microscopist, for his work as an
experimenter, allied to hisunequalled
dexterity as a section cutter and a dissector of minute organisms, give him
additional status as a pioneer of eighteenth century experimentalbiology.

REFERENCES

1: A summary of earlier attitudes towards Leeuwenhoek's working methodsappeared in Ford, Brian J., The van Leeuwenhoek Specimens, Notes and Records of the Royal Society, (1981) 36 (1), 37-59. The review published as Folkes,M., Some Account of Mr Leeuwenhoek's Curious Microscopes, lately Presented totheRoyal Society, Philosophical Transactions, (1724) 32,446-453, implies -even its title - some degree of condescention. L. M. Becking dismisses Leeuwenhoek as "immortal dilettante" in Science Monthly, New York, (1924),18, 547; and J. Sachs (1875) regarded him as inconsequential in his History of Botany, London & Munich.

3: Transcriptions of the letters to the Society, and to Leeuwenhoek's othercorrespondents, are found in The Collected Letters of Antony van Leeuwenhoek,Allede Brieven van Antoni van Leeuwenhoek, Uitgegeven ... door eenCommissie van Nederlandschegeleeren), Swets & Zeitlinger, Amsterdam, 1939-present. It is anticipated that the considerable volume of correspondencewill be published in English and Dutch by the end of the twentieth century.It may be noteworthy that the time taken for the publication of the correspondence is a decade longer than it took Leeuwenhoek to originate it.

4: It has been widely believed that all Leeuwenhoek's original materials werelost, indeed the survey by Bracegirdle published prior to this revelationstated emphatically that "no preparations from the seventeenth century hadsurvived." The account (in B. Bracegirdle, 1978, A History of Microtechnique, Ithaca: Cornell), adds that any such specimens would have been poorlyprepared in any event, and unlikely to reveal much of microscopical interest.However, the discovery that nine specimen packets had survived amongstLeeuwenhoek's correspondence has proved to disclose that he was an excellent microtomist. An analysis of the material is in press, Brian J. Ford, The Leeuwenhoek Legacy, Farrand and Biopress, London, 1991 and an early appraisalwas published in 1981 as Notes and Records of the Royal Society, 36 (1), 37-59. Brief notes may be found in Nature, 1981, 292, 407 and New Scientist,1981, 91, 301.

5: An informal account of Leeuwenhoek's early days was published by ElmerBendiner, The Man who did not invent the Microscope, Hospital Practice,August 1984, 139, 144-160,165-174.

6: Henry Baker reproduced Leeuwenhoek's account of this coelenterate in hisMicroscope made Easy of 1743. His figure III p 94 is a reproduction of thefigure 3 in Leeuwenhoek's account, published in Philosophical Transactions No283, 1703. Interestingly, Leeuwenhoek's original submission had shown adiagram of Hydra with eight tentacles, increased by the engraver to nine in Philosophical Transactions, q.v. This was corrected to eight in the Bakerversion, strongly suggesting that he referred, not to the published version but to Leeuwenhoek's original correspondence in drawing up his account.

10: Leeuwenhoek's "outer layer" is the seed coat proper; the soft membrane isthe endosperm which surrounds the embryo itself. Leeuwenhoek is wrong torefer here to "four distinct leaves", however. Within the cotton seed are twocotyledons. These are folded against each other and are liable to breakunless handled with extreme care. It is probable that his manipulations separated the convoluted structures into four separate parts. His descriptionof "small spots" is an example of acutely accurate observational microscopy; these are the glandular structures on the leaves which contain brown orviolet secretions.

11: The term Leeuwenhoek used in his letter is Vlijm, closest perhaps to 'phlegm' in English. Note too that Leeuwenhoek described the conditioning ofthe City water courses by means of sluice gates in the letter of 9 October1676, Collected,Letters ... II, 85.12. This view is examined by P. van der Star in: Intellectuals Against Leeuwenhoek, (in) Antoni van Leeuwenhoek, eds: L Palm and H Snelders,Amsterdam, 1981.